The present invention relates to image rotation of scanning displays, and more particularly to circuitry for rotating the image on a cathode ray tube or simialr raster-driven display device by electronically rotating the deflecting circuits. Most specifically, the present invention relates to a system for rotating a helmet image display to compensate for tilting of the helmet wearer's head.
Raster video images are a standard feature of most aircraft in use today. In recent years, it has been found desirable in some instances to mount such displays on the pilot's headgear, or helmet, so that the display is always in his field of view. Such helmet displays may incorporate a small cathode ray tube or other similar display screen where a raster scanning type display provides a radar image for easy viewing by the pilot. Such displays present problems, however, if the image is not in register with the background which is within the remaining field of vision of the pilot. This lack of register can occur, for example, when the pilot tilts his head and causes the display unit also to tilt with respect to the horizon. In existing displays, this would result in tilting of the image of the horizon with respect to the actual horizon, and this can confuse or disorient the pilot. What is needed, therefore, is a technique for adjusting the image so that it remains in alignment with a reference such as the horizon or some other background object, so that when the pilot tilts his head, the image will remain in synchronism with the background.
Various methods of rotating images are known in the art, but methods such as the use of scan conversion, can be expensive in terms of size, weight and power, as well as in terms of dollars. The use of stroke type displays, also known variously as random access or vector displays, is viable only if the display is limited to symbology, and no images are displayed. Further, the recording of stroke displays is inconvenient. Thus, there is a need to develop an inexpensive, low power, accurate method and circuitry for electronically rotating a scanned image.